The use of interferometry to measure changes in lengths, distances, and optical paths is well known in industry. Collectively such practice can be termed interferometric displacement measurement. In performing such measurement both homodyne and heterodyne techniques may be used, with the latter having come to be overwhelmingly preferred today. Of present interest are heterodyne interferometry techniques using two optical frequencies, and preferably using frequencies produced with single laser devices.
In general, in a single mode laser only one frequency of oscillation may be produced. In order to allow more than one frequency to oscillate simultaneously, new boundary conditions have to be introduced into the laser resonator so that more than one gain media is formed. The application of a magnetic field to at least part of the laser gain media is one well known way to accomplish this, and inserting a photoelastic material into a laser cavity to create birefringence and produce different optical paths is another.
When a magnetic field is applied to a single longitudinal mode laser cavity two oscillation frequencies may be produced which have orthogonal polarizations and are separated in frequency symmetrically with respect to the absolute frequency of the laser (its natural resonant frequency). This is commonly termed the Zeeman effect, and lasers using it are called Zeeman lasers. In a Zeeman laser the magnetic field may be applied along the same direction as the axis of the laser resonator (axially or longitudinally) or perpendicular to the axis of to the laser resonator (transversely).
For axial type Z eeman lasers the frequency components produced have opposite circular polarizations and maximum frequency split is typically a few megahertz, e.g., for He--Ne lasers approximately 4 MHz. For transverse type Zeeman lasers the frequency components produced have opposite linear polarizations and the maximum frequency split is typically only a few kilohertz, e.g., for He--Ne lasers approximately 300 KHz. He--Ne lasers are used herein as examples. However it should be appreciated that the Zeeman effect may be obtained in other laser mediums and that the present invention may therefore also use such alternate mediums.
The split dual frequencies obtainable with Zeeman lasers are particularly useful for interferometric displacement measurement using heterodyne techniques. A key benefit is that the Zeeman split is symmetric with respect to the absolute frequency, which can be determined very precisely for the particular laser medium used. It follows that the frequency for each frequency component can also be precisely determined. Zeeman lasers also achieve high signal-to-noise ratios. In interferometric displacement measurement these characteristics permit the interference fringes produced by the motion of a target object to be accurately measured, and the total displacement of the target may be calculated by integrating the total number of such fringes through time. This method of displacement measuring is accurate and reliable, and has found wide use in industry.
One way to increase the frequency split produced by Zeeman type lasers is to apply a stronger magnetic field to the laser resonator. However, there are practical limits to this. As the magnetic field is made increasingly strong a point is reached at which the gain media starts to behave in a non-linear fashion, and second order Zeeman effects then cause unwanted modes and frequencies to appear. This confuses the detectors used in interferometer systems. Overly strong magnetic fields also push the gain of the media away from the absolute frequency, dramatically decreasing the laser power produced, until the point at which lasing stops entirely. Thus, there is an upper limit to the frequency split obtainable using the Zeeman effect.
In displacement interferometry this the maximum obtainable frequency split imposes a limit on target speed during measurement (velocity=2 * wavelength * Doppler frequency). For example, if a measurement target object is moved such that the Doppler effect causes a decrease in the frequency split, the measured frequency can decrease all the way to zero and the interferometer can cease to function. For axial He--Ne Zeeman lasers the maximum target movement rate, commonly called the "slew rate", is approximately 1.2 m/sec. For transverse He--Ne Zeeman lasers the maximum slew rate is considerably less (&lt;0.1 m/sec). Today axial He--Ne Zeeman lasers are widely used in industry, but it is becoming increasingly desirable to perform displacement measurement using still higher slew rates.
Other techniques than the Zeeman effect can also create multiple frequencies. One well known example is insertion of a photoelastic material into a laser cavity to add birefringence. However, such other techniques generally also suffer a common shortcoming: they have a minimum obtainable frequency split of approximately 40 MHz, which is simply not practical for use in most current interferometry applications. Thus, current techniques are not able to produce split dual frequencies fi)r interferometry in a range extending roughly from 4 MHz to 40 MHz.
Accordingly, new techniques for achieving split dual frequencies for interferometric measurement are needed, particularly ones which produce frequency splits in the range from 4 MHz to 40 MHz.